1. Field of the Invention
This invention relates to the electrodeposition of chromium for functional purposes on basis metals from hexavalent chromium plating baths. More particularly, it is concerned with chromium baths which are capable of producing advantageous chromium deposits at high efficiencies and high temperatures without low current density etching.
2. Description of the Prior Art
Typical hexavalent chromium plating baths are described in U.S. Pat. Nos. 2,750,337; 3,310,480; 3,311,548; 3,745,097; 3,654,101; 4,234,396; 4,406,756; 4,450,050 and 4,472,249. These baths are generally intended for either "decorative" chromium plating or for "functional" (hard) chromium deposition. Decorative chromium plating baths are concerned with deposition over a wide plating range so that articles of irregular shape can be completely covered. Functional chromium plating baths, on the other hand, are involved with regularly shaped articles where rapid plating at a high current efficiency and at useful current densities is important.
Functional hexavalent chromium plating baths containing chromic acid and sulfate as a catalyst generally permit the deposition of chromium metal on the basic metal at cathode efficiencies of between 12% and 16% at temperatures between about 52.degree. C. to 68.degree. C. and at current densities of from about 30 to about 50 a.s.d. Mixed catalyst chromic acid plating baths containing both sulfate and fluoride ions generally allow the plating of chromium at higher rates and at cathode efficiencies of between 22% and 26%. The presence of fluoride ion in the bath, however, causes etching of ferrous based metals when the cathode current density is too low to deposit chromium metal, usually below about 5 a.s.d. in fluoride containing baths. This phenomenon is referred to as "low current density etching". Additives for chromium plating baths to prevent low current density etch are described in U.S. Pat. Nos. 2,750,337; 3,310,480; 3,311,548; and 3,654,101. Unfortunately, these additives severely limit the current efficiency of the process.
Some chromium plating baths are designed to impart a decorative iridescence to the deposit. Such baths include hexavalent chromium metal ion, a first additive composition, such as a haloalkyl sulfonic acid or haloalkyl phosphonic acid, and a second additive composition which is a carboxylic acid. The simultaneous action of these two additives in the bath produce the desired iridescent effect. However, there is an accompanying substantial reduction in the current efficiency of the process with these baths.
Other chromium plating baths which use iodide, bromide or chloride ions as additives can operate at a high current efficiency; see U.S. Pat. Nos. 4,234,396; 4,450,050; and 4,472,249; but such baths produce chromium deposits which do not adhere well to the substrate, and which are dull in appearance at high plating temperatures, or only semi-bright when formed at low plating temperatures.
Accordingly, it is an object of the present invention to provide a chromium plating bath for producing non-iridescent, adherent, bright chromium deposits at high cathode efficiencies and at high plating temperatures which are substantially free of low current density etching.
Another object of the invention is to provide a process for producing such advantageous chromium deposits under useful plating conditions.
These and other objects will be made apparent from the following more detailed description of the invention.